OUTLIiNES OF THE GEOLOGY OF TASMANIA. By W. H, TwELVETREES, F.G-.S., Governnumt Geologiat. Tasmania, a small geological outlier of Eastern Australia, offers a highly interesting field to the geological student. It must, however, be conceded that its physical histoiy in pre-Cambrian and early Palaeozoic times can only be dimly guessed at. In later Palaeozoic times, the conditions appear to have been insular; in the Mesozoic, there was evidently a connection with the great Gondwana continent, which sank beneath the Indian Ocean prior to the Tertiary period. The greater part of the island has since remained above sear level. The inaccessible nature of the highlands has greatly re- tarded geological research ; still, in spite of the physical diffi.culties, the progress made in this direction has been con- siderable. In 1841-5, Count Strzelecki published some geological notes on the Island. From 1851 to 1855, Dr. J. Milligan, then Secretary of the Royal Society of Tasmania, reported on a large portion of the East and South Coasts. In 1855, Mr. A. R. C. Selwyn reported on coal seams. In 1861-7, Mr. Chas. Gould, Government Geologist, prepared numerous important reports and maps. Mr. S. H. Wintle contributed various geological notes, 1865-1882; Rev. J. C. Tenison- Woods has written several papers on Tertiary geology and palaeantology ; Mr. C. P. Sprent, Deputy Sur- veyor-General, from 1876 to 1887, wrote on the Weste-rn geology of the Island; Mr. Thos. Stephens, M.A., from 1863 to the present date, has constantly contributed to our know- ledge of the geology of the State; in 1888, Mr. R. M. John- ston's monumental work, " The Geology of Tasmania,"- ap- peared under Government auspices, and for many years this author has enriched our geological literature. The late Pro- fessor G. H. F. Ulrich, in 1874-6. reported upon Mounts Bischoff and Ramsay. Our Government Geologists, Messrs. G. Thureau, Alex. Montgomery, M.A., and Mr. Jas. Har- court Smith, B.A., have, in no mean degree, extended our
Transcript
OUTLIiNES OF THE GEOLOGY OFTASMANIA.
By W. H, TwELVETREES, F.G-.S., Governnumt Geologiat.
Tasmania, a small geological outlier of Eastern Australia,
offers a highly interesting field to the geological student.
It must, however, be conceded that its physical histoiy in
pre-Cambrian and early Palaeozoic times can only be dimly
guessed at. In later Palaeozoic times, the conditions appear
to have been insular; in the Mesozoic, there was evidently
a connection with the great Gondwana continent, which
sank beneath the Indian Ocean prior to the Tertiary period.
The greater part of the island has since remained above sear
level.
The inaccessible nature of the highlands has greatly re-
tarded geological research ; still, in spite of the physical
diffi.culties, the progress made in this direction has been con-
siderable. In 1841-5, Count Strzelecki published somegeological notes on the Island. From 1851 to 1855, Dr. J.
Milligan, then Secretary of the Royal Society of Tasmania,
reported on a large portion of the East and South Coasts.
In 1855, Mr. A. R. C. Selwyn reported on coal seams. In1861-7, Mr. Chas. Gould, Government Geologist, prepared
numerous important reports and maps. Mr. S. H. Wintlecontributed various geological notes, 1865-1882; Rev. J.
C. Tenison-Woods has written several papers on Tertiary
geology and palaeantology ; Mr. C. P. Sprent, Deputy Sur-
veyor-General, from 1876 to 1887, wrote on the Weste-rn
geology of the Island; Mr. Thos. Stephens, M.A., from 1863
to the present date, has constantly contributed to our know-ledge of the geology of the State; in 1888, Mr. R. M. John-
ston's monumental work, " The Geology of Tasmania,"- ap-
peared under Government auspices, and for many years this
author has enriched our geological literature. The late Pro-
fessor G. H. F. Ulrich, in 1874-6. reported upon MountsBischoff and Ramsay. Our Government Geologists, Messrs.
G. Thureau, Alex. Montgomery, M.A., and Mr. Jas. Har-court Smith, B.A., have, in no mean degree, extended our
GEOLOGY OF TASMANIA. 59
knowledge of the general geology of the Island. Mr. W. F.
Petterd has contributed his " Catalogue of Minerals of Tas-
mania," and several papers on the eruptive rocks. Otherworkers, Professors David, Tate, Krause, Hogg, Feistmantel,
Mr. R. Etheridge, Jun., Mr. J. Dennant, have added the re-
sults of their researches, while the names of authors of
papers read before the Royal Society of Tasmania (Messrs.
W. F. Ward, Alex. Morton, Danvers Power, T. B. Moore,Graham Officer, (kc.) suffice to show that this Society has
had an honourable share in the construction of the literature
of the subject.
Mr. R. M. Johnston, the doyen of Tasmanian geologists,
has worked out thoroughly the stratigraphy of the Tertiary,
Mesozoic, and Upper Palaeozoic systems, and successive
Government Geologists have contributed to our knowledgeof detached areas in different parts of the Island ; but the
lower Palaeozoics still require much study before they can
be properly defined. In this sketch, the main develop-
ments of each system, as far as at present known, will bebriefly referred to.
Pre-Camhrian
.
The massive quartzites at Port Davey are usually referred
to^ this age, but their stratigraphical relations need workingout. The mica-schists and gneiss-like rocks at the DoveRiver, and the hornblende zircon-gneiss of the Upper Forth,
are also possible members. In the North-West, the horn-
blende and talc schists, with associated dolomitic limestone
in the Rocky River district, enclosing deposits of pyrrhotite
and copper pyrites, need investigation. These rocks are
v/ell seen at the Rocky River Mine, and at the Rio Tinto,
further north, on the same strike. The hornblende schist
runs through to the junction of the Nine-mile Creek with
the Whyte River.
Cambrian.
The only strata which can be definitely referred to the
Cambrian system are the friable yellow sandstones at Caro-
line Creek, between Railton and Latrobe. These have a
strike (E. 60° S.) different from the prevailing direction of
the Silurian strata of the Island, and contain Dikelocephalifs
Eth., Jun.), Asaphus sp., Scolithiis tasmanicus (R. M. John-ston), Leptaena. These are the most ancient fossils yet
found in our rocks. The elucidation of the relations of
these strata with the adjacent schists and limestones is muchneeded.
60 gec»lo(;y of Tasmania.
Silurian.
The divisions of this system are still largely tentative.
The following scheme, in which the eruptive rocks of theperiod are included, must be taken as provisional :
—
Upper and Middle SUtirian.
5. Quartz porphyries and felsites at Mounts Darwin,Jukes, Ovx'en, Tyndal, Read, Red Hills, Black,Murchison, Farrell
;
4. Gabbros, peridotites, pyroxenites, and serpentine at
Dundas, Trial Harbour; Heazlewood, Forth,
Anderson's Creek
;
3. Brachiopod sandstone, at Middlesex, Heazlewood,Queen River ; slates, sandstones, and limestones,
with meiaphyre lava, at Zeehan
;
2. Schists, conglomerates, and limestones, at MountLyell ; greywacke series at Dundas ; slates andargillaceous schists, at Mounts Read and Black.
Lower Silurian.
1. Limestone, at Gordon River, Railton, Chudleigh,&c. ; slates and sandstones, at Beaconsfield, Lefroy,Mangana, Mathinna, Scamander, &c.
The Silurian system is strongly developed in Tasmania,especially in the N.E., N.W., and Vv^. Owing to paucityof fossils, its subdivisions are unreliable, except in a fewinstances, and its boundary-lines with the Cambrian rocksare still obscure. The lower division is represented on theWest Coast by the Gordon River series, and on the Eastby the slates, in which our gold reefs occur at Lefroy,Beaconsfield, Mathinna, &c. The limestones along theGordon River are fossiliferous, containing Favosites, Ortho-ceratites, Ra'phistoma, Orthis, Rhynchonella, Euomphalus,Mvrchi^onia,'kc. They reappear to the N.E. of MountFarrell in the bed of the Mackintosh, a short distance aboveits junction with the Sophia River. The limestones of
Chudleigh, Mole Creek, and Ilfracombe are plaxed provision-
ally in the lower division. They are non-fossiliferous, andthe only way of fixing their age is to connect them strati-
graphically with the Caroline Creek Cambrian beds. Theslates and schists between the Heazlewood and Corinna be-
long to an undetermined horizon in the system, and someof them may be pre-Silurian. The slate and schist reefs
which run out to sea on the N.W, Coast can onlyvaguely be referred to as Silurian ; at Rocky Cape, they areprobably lower in the geological record.
GEOLOGY OF TASMANIA. 61
The auriferous slate series, with sandstones and con-
glomerates, appear at Beaconsfield, Lefroy, Waterhouse,
Gladstone, Mount Victoria, Mathinna, Scamander, Fingal,
&c. Fossils are extremely rare. They comprise doubtful
fucoid casts, worm tracks, and, in one instance, a graptolite
is recorded from the Lisle slates. The specimen was found
by the late Mr. G. Thureau, and subsequently lost ; but,
from inquiries, it seems nearly certain that it was a Diplo-
graptus. Unfortunately, the range of this genus is too
great for use in deternniuing the horizon of the beds. Themetamorphic sandstones of the St. Helens and Scamanderdistricts are referred doubtfully to the same horizon as the
slates.
It is difficult tO' locate the so-called schists (slates; and ar-
gillites) of Mounts Read and Black. These are charged
with complex gold and silver-bearing sulphidic ores of zinc,
lead, and copper. They may be low down in the system;
or, on the other hand, they may be contemporaneous with
the Lyell schists. The latter also cannot be placed de-
finitely, but, from fossil brachiopods found at Gormanston,
it seems possible that they belong to the Queen River
series, greatly metamorphosed. The King and Queen River
slates and sandstones, charged with fenestellidas and en-
crinites, and casts of brachiopods (spirifera and orthis), be-
long to the Middle Silurian or the lower part of the UpperSilurian. Silurian sandstones at the Heazlewood, tov/ering
above the road at the 14-mile, and on the old bodkin Amal-gamated, are referred by R. Etheridge, Juu., to the lower
part of the Upper Silurian. They have yielded the follow-
formis sp. nov. (in the Despatch limestone), Lophospira (in
quartzite), Miirchisonia (in quartzite), Eunema montgomerit(in the Despatch limestone), TentacuUtes sp. nov. (in slate),
Raphistoma (?) sp. nov. (in white sandstone).
The general trend of the Zeehan beds is west of N. andeast of S., and their dip is to the N.E. at angles of from60^ to 70O. It may be mentioned that a high angle of dip
62 GEOLOGY OF TASMANLA.. '
characterises the Silurian strata throughout the Island.
Interbedded with the sedimentary beds at Zeehan are sheets
of Silurian basalt (melaphyre), known locally as '' whiterock." This is often tuffaceous and vesicular. In the
Oonah and Montana mines, it may be seen in the form of
contemporaneous sheets.
Of about the same age are slates, sandstones, and lime-
stones in the Bell Mount district, between the Forth andWilmot rivers. The sandstones there and at Mount Claudecontain abundant casts of fucoid stems; fenestella, trilo-
bites, and rhynchonella also occur at Bell Mount and the
Five-mile Rise. Clay slates, with calymene, orthis, car-
diola, in the Eldon Valley, are referred to the UpperSilurian.
Associated with the rocks of the system in the N. andW. is an extensive development of serpentine, the altered
form of gpvbbro and its appendages, peridotite and pyroxe-nite. Dykes of it cross the Silurian strata on the roadbetween Waratah and the Whyte Biver, and the rock
underlies metamorphosed sandstones at the Heazlevv^ood.
A great variety of gabbros and pyroxenites may be seen
along this road. Nickel Hill, at the Sixteen-mile, is a massof serpentine rock, containing nickel ores, and Bald Hill,
immediately to the west, is likewise serpentine as far as the
Nineteen-mile, where it impinges against Silurian slates.
A pyroxenite dyke in Silurian strata carries the silver-le-ad
lode at the Magnet Mine. Gabbro, pyroxenite and ser-
pentine occur in the Dundas district, and reappear west of
the Comstock, and again at Trial Harbour. In the Valleyof the Forth, and at Anderson's Creek, west of Beaconsrfield, further areas of serpentine are exposed, and at the
latter place the rock is often asbestiform, and is mined for
asbestos. It is difficult to assign a precise age to ourgabbros and serpentine. They have been thought to bepre-Silurian ; but the Heazlewood intrusions suggest the
close of the Silurian as a possible date.
Very important rocks are the quartz-porphyries, or fel-
sites, which form the backbone of the West Coast Range.These are the geographical axes of Mounts Darwin, Jukea,Huxley, Tyndal, and continue northwards through MountMurchison, and on the east side of Mount Farrell. Theyare the home of copper ores, and enclose characteristic de-
posits of hematite and magnetite. Chloritic copper-bearing
schists, some of them probably schistose porphyries, flank
them, and are enclosed in them. On the whole, the quartz-
porphyry is massive, but it occurs also laminated. It wa3probably intrusiyei, but this can only be decided after
GEOLOGY OF TASMANIA.'
63
further investigation. Its tendency to assume laminated
forms indicates that it was involved in the foliation of the
Silurian rocks. Its connection with our granites has not
been worked out. It is placed ^vith some hesitation at
the close of the Silurian.
A belt of felsite, a little to the west of this zone, can be
traced through Mounts Read and Black, across the PiemanRiver, at the raihvay crossing. The green augite-syenitic
rock at Lynchford has probably some connection with the
felsites.
Devonian.
3. Dial Range and West Coast upper conglomerates.
2. Soft slates at Fingal.
1. Granite in North, East, and West Tasmania.
Our granites are considered to be of Lower Devonian age,
i.e., soon after the close of the Silurian. No granite intru-
sion into Permo-CarT)oniferous strata has been observed,
while it is frequently intrusive into the Lower Silurian
slates, and has been established as intrusive into UpperSilurian at Middlesex. Evidence has been forthcoming
recently, at the Heazlewood and at Mount Agnew, showingthat the consolidation of the granite was subsequent to that
of the gabbroid rocks. There is an exposure of granite,
generally tin-bearing, running down the eastern side of the
island from Mount Cameron and Mount Stronach to the
Blue Tier and Ben Lomond, St. Marys. Seymour, Bicheno,
Freycinet's Peninsula, Maria Island, as far south as the
Hippolyte rocks. It occurs again in the Middlesex Field,
at Granite Tor, Mount Farrell, Hampshire Hills, MountHousetop, Magnet and Meredith Ranges, Mount Heems-kirk, Mount Darwin, and evidently underlies the whole of
the West Coast. The quartz-porphyry dykes at MountBischoff, the tourmaline lodes at Mount Black, Renison Bell,
and elsewhere in North Dundas, the staunite lode andspherulitic quartz reef at Zeehan denote the granitic reser-
voir below a large portion of the mineral fields on the WestCoast. The normal granite is a dark mica one, mostly
spotted with large porphyritic crystals of orthoclase felspar.
In its tin-bearing varieties the magnesian mica disappears,
The Fingal slates, of a soft sandy nature, have beendoubtfully retained in the Devonian, on the strength of a
fossil resembling Anodonta jtikesii; but it is uncertain
whether they can be stratigraphically separated from the
Silurian slates at Fingal.
(>4 geolo<;y ov iasmama.
Tlie horizontal bods of longlomcrato, whiih lie as lieavy
caps on the Dial Range an«i most of tho Western Moun-tains, have been assigned to this system. These massiveconglomerates crown IMount FarrcU. Mnnhison, Lyell.
C)\ven. Jukes, Ixoland, Claiule. A-c.
^'(rrno-Cdrhonifttdus.
These rocks consist of sandst^mes, mudstones, grits, con-
glomerates, and' limestones, with shales and thin coal seams.The most productive coal measures in TaLinania do not be-
long to this system, but are Upper Me^ozoic, probablyJurassic. The Permo-Carboniferous strata have beenthoroughly examined by Mr. R. M. Johnston, and his classi-
fication is adopted:—rp2^tr—
7. EliTolite syenites, phonolites and trachytes, at PortCygnet.
6. Southpoi-t, sandstones and shales.
5. Mount Cygnet and Adventure Bay, sandstones andshales.
and grit-s. throughout S.E., N.E., and Midlands.Conglomerates, grits, and micaceous sandstones and slaty
llagstones, in thick beds, form the base of the system. Theseconglomerates, at One Tree Point, North Bruni, at Darling-
ton, the northern point of Maria Island, below the lime-
stone beds, contain large blocks of granite, porphyry, ilec.
The angular blocks on Maria Island are over a ton in
weight, and on Bruni, too, they are very large. The LowerMarine series of limestones and mudstones comprises, in
Southern Tasmania:—3. Fenestella mudstones. at Porter Hill, The Grange.
Cascades, etc.
'2. Spirifera and strophalosia mudstones, Huon Road,i\:c.
1 . Limestones, on Mai'ia Island, at Bridgewater ; also
at Fingal. Middle Arm, etc.. in the North. Theycontain favositcs, spirifera, product us, conularia,
pdchi/domus, notomya, aviculopecfen, &c.
These marine beds occur all along the Derwent, from BruniIsland to New Norfolk. At Porter Hill, south of the Alex-andra Battery, on the Brown's River Road, sections are ex-
posed of the lower marine series, with its common fossils^
GEOLOGY OF TASMANIA. 65
passing upwards into shales and sandstones of the upperdivision of the system, with Gangamo'pitru and Cytheretasmanica (Johnston).
Fossiliferous limestones and mudstones occur at Variety
Bay. At Eaglehawk Xeck. the sea beach exposes grits andconglomerates with rectangular joints filled with oxide of
iron, forming a natural " tesselated pavement " greatly ad-
mired by visitors. The jointing is probably due to the
vicinity of a concealed body of intrusive diabase. At the
Middle Arm of the Tamar, near Beaconsfield, the fossil-
iferous limestones repose on Silurian rocks. Dally- old
quarry abounds with Eurydesma cordata. Fossiliferous
mudstones, with spirifera, productus, terebratula, pachy-domus, eurydesma, occur on the Meander, near Cheshunt.At Mount Cygnet, the succession is—3. fenestella zone; 2,
spirifera zone ; 1, shaly mudstones. The spirifera sand-
stones occur all round Lovett and Lymington.On the West Coast, the lowest conglomerates of the
system are composed of pebbles of schist and quartzite, andrest on ancient schists in the Barn Bluff district.
The upper division of the system comprises sandstones andshales, which contain the coal of the period, and includes
marine mudstones, overlying the coal in the Mersey dis-
trict. In the Mersey Basin, notably, near the Great Bendof the river, near Latrobe, beds of variously-coloured clays
enclose thin layers of bituminous shale, called Tasmanite.from the abundance of fossil spore cases of the lycopod Tas-
tnanites punetatus (Newton), which contains over 25% of
resinous matter. The exact relation of these shales to the
other beds in the Mersev Basin is not settled.
The beds of the Mersey coal measures are grits, varie-
gated sandstones, marls, and the coal plant remains are the
forms characteristic of the Permo-Carboniferous, viz. :
—Glossopterisy Gangamopt eris spatulata, G. ohliqya, Noegger-athiopsis media. Mr. Johnston has also recognised a schizo-
neiira (rare). The coal of these measures is superior in
quality to the coal in the Jurassic measures, but the seamsare not of such importance. They are overlaid by marinemarls and limestones, sandstones, and conglomerates, withFenestella pleheia, Spirifera tasmaniensis. Terfhratula sac-
culiis, Pleurotomaria morrisiana, Pachydomus, Aviculopec-ten, Cardiomorpha, Pter'ma, &c. These are called theUpper Marine Beds in Tasmania.The upper zones of sandstones and shales at Porters Hill,
in the South, correspond with the Upper Marine beds of theMersey. Two hundred feet of the former are exposed alongthe Derwent, containing Cythere tasmanicu^ (Johnston),
66 GEOLOGY OF TASMANIA.
Gangamo'pteris, Sfirifera tasmaniensis, S. darwinii, S. duo-decimcostata, Terehratula saccidus, Avicida, Area, Aviculo-
On the north bank of the Henty River, on the WestCoast, between the Henty and Badger, the lower coal
measures are hard dark grey shales, which contain Gangam-opteris spatulata (McCoy), G. ohliqua (McCoy), Noeggera-thiopsis media (Ettingsh.). Above these are mudstonesand impure limestones, with Fenestella plebeia, F. internata,
Protnretepora ainpla, Stenopura tasmaniensis.
In the North-East part of the!\ Island, foraminiferal lime-
stone of this system has been found by Mr. Thos. Stephens.
At Harelield, in the Fingal Basin, a diamond-drill bore hasrevealed the existence of 97 feet of conglomerates, sandstone,
and shales, resting on Silurian slates, at a depth of 674 feet
in the bore-hole. These underlie the Upper Marine beds.
Very little coal was found, but the shales contained im-
prints doubtfully referred to Schizoneura and Gangamop-teris. The Upper Marine beds overlying these were 313feet thick, and consisted of fossiliferous blue shale, lime-
stones, mudstones, &c.
At Mount Cygnet, the low^er coal measures rest on the
fenestella beds, and are overlaid by 200 feet of grey sand-
stone. The coal shales contain impressions of Vertehraria
australis and Gangamopteris spatulata.
At Adventure Bay, on Bruni Island, lower coal measureshales and seams lie conformably on the lower marine mud-stones, conglomerates, and sandstones. They contain
dwarfed forms of Gangamopteris spatvlata, G. ohliqua, Gloss-
opt eris hrowniana, var. prcecursor (Brongt.).
At Southport, brown sandstone is overlaid by carbon-
aceous shales, with imprints of Vertehraria australis. TheAdventure Bay and Southport series form the uppermostbeds of the system.
The elseolitic and trachytic rocks, which are developed at
Port Cygnet and Oyster Cove, are referred provisionally to
the close of this period. Some of them appear to be fluidal,
and interbedded with the Permo-Carboniferous mudstonesand sandstones, but further examination is requisite. Themajority are intrusive rocks, forming parts of a mass of
elseolite-and alkali-syenite, M^th associated dykes of phono-litic, tinguaitic, and trachytic porphyries. The accessory
minerals of the nepheline rocks, nosean, aegirine, sodalite,
melanite, &c., are present here in all the wonderful variety
characteristic of that group. Mounts Livingstone and
GEOLOGY OF TASMANIA. 67
Mary, on either side of Lovett, and the beach south of the
Eegatta Ground, show these rocks in great variety. A good
deal of free gold has been shed into the alluvial flat at
Lymington. The source of the metal is believed to be the
line of contact between the porphyries and the Permo-Car-
boniferous sediments. This belt of rock passes S. to
the other side of the Huon River, and N. across to Oyster
Cove.Mesozoic.
^
The series of freshwater beds which succeed to the UpperPalaeozoic belongs to the Mesozoic division, but cannot, as
yet, be subdivided with certainty. The nearest approach
to a subdivision would be as follows ; but the reference to
European equivalents is quite provisional :
—Cretaceous ( ?)
—
4. Diabase (dolerite) in intrusive masses, laccolites, sills,
and dykes.
Jura—3. Upper coal measure sandstones.
Trias—2. Sandstones and shales, with coal seams, at Ida Bay,
containing Fecopteris lunensis (R. M. Johnston).
1. Variegated sandstones, with Vertehraria australis
(McCoy), and remains of heterocercal fishes andamphibians.
1. The sandstones at the Government House Quarry, in
the Domain, at Knocklofty, at Ross, &c., belong to the
Lower Mesozoic. Mr. R. M. Johnston considers the LowerSandy Bay mudstones, exposed three miles from Hobart, on
the Brown's River Road, to be the base of the system.
They conta,in obscure plant impressions. The variegated
sandstones of Lower Sandy Bay are supposed to overlie
them conformably. In the Domain, the sandstone has
yielded bones of amphibians {Lahyrinthodonts X). Fromthe Cascades to Knocklofty, there are about 1000 feet of
these sandstones, from which the heterocercal fi^,Acrole'pis
hamiltoni (Johnston and Morton), has been recorded.
Messrs. Johnston and Morton give the section in ascending
order, as follows :—
Feet.
1
.
Yellow fissile sandstone 202. Flaggy sandstone, with fish remains 5
3. Mottled shales, with plants 604. Thick sandstone beds, quarried for bui'd-
ing ' 715
800
68 GEOLOGY OF 'J'AS.MANIA.
The sandstone near Tinderbox Bay is on the same horizon
as the Knocklofty beds, and contains remains of a fish de-
scribed by Messrs. Johnston and Morton, under the nameof Acrolepis tasmanicus. This sandstone overlies conform-ably the uppermost beds of the Permo-Carboniferous mud-stones.
This series of sandstones and shales contains the plant
remains called Vertehraria australis (McCoy). Recently,
Vertebraria has been regarded as the rhizome of glossopteris.
In Tasmania, it is confined to the Lower Mesozoic, and the
passage beds at Southport, which are just the strata in
which glossopteris has not been found.
3. The sandstones which enclose the Mesozoic coal seamsare readily recognised by their soft, felspathic nature; theyare generally greenish-grey to yellowish brown, some-times
white. They are widely spread throughout East andSouth-East Tasmania, and occur also in the South. Themaximum development obsei^ed is about 1000 feet. Theyare largely interrupted by intrusions of diabase, whichbreaks through, and, to all appearances, locally overspreads,
them. Whether this overspreading is real, or only apparent,
is still a matter of dispute. They flank the Central,
Western, and Eastern Tiers, and fringe isolated mountaincaps of diabase at Mount Nicholas, Mount Victoria, MountSaddleback, Ben Nevis, Mount Elephant, Mount Dundas,Cradle Mountain, Ben Lomond, Tower Hill, &c.
From Fingal and Mount Nicholas they extend on the
outskirts of the diabase ranges southv/ards to Seyn).our,
Douglas, and Denison rivers, Llandaff, Spring Bay, andthence all over South-East and a good deal of South Tas-
mania, besides encircling the whole of the elevated central
part of the Island with a narrow girdle. In the South-East
they are cut up very much by intrusive diabase. In this
brief description detailed mention of localities is impossible.
Well-known occurrences are those on Ben Lomond, SchoutenIsland, Triabunna, Okehampton, New Town, Sandfly
Rivulet, Tasman Peninsula, Upper Derwent, Campania,York Plains, Norwich, &c. The fossil fl.ora from these beds
must be regarded as characteristic for the Upper Mesozoic.
The plants have been scheduled by Mr. R. M. Johnston, as
follow :—
Fdices—Alethopteris Australis (Morris)
„ serratifolia (R. M. Johnston)Cardiopteris Tasinanica „Cyclopteris ? Australis (possibly a
Salisburia) = „Danaea Morrisiana ,,
GEOLOGY OF TASMANIA. 69
Gleichenia dubia (M'Coy)Glossopteris moribunda (R. M. Johnston)Maorotaeniopieris Wianaraattae ( Feiston)
Neuropteris antipoda (R. M. Johnston)
„ Tasnianiensis „Odontopteris crispata
,,
Peoopteris Buftoni ,,
„ caudata ,,
„ odontopteroides (Morris)
Rhacophyllum coriaceum (Ti. M. Johnston)Sagenopteris Tasmanica (Feiston)
„ strahani „Sphenoza mites teistmantelii „Ptilophyllum oligoneurum (T. Woods)
Co)iiferae.
Baiera tenuifblia (R. M. Johnston)Ginkgophylhim austraHs
?»
Salisburia hobartensis,,
Zeugophy Kites (poa-cordaites) elon-
gatus „
The sandstones are extensively broken by. intrusions of dia
base, or dolerite, which cut up the coal measure areas into
different basins. Dykes of diabase traverse the beds.
This rock, called dolerite in England and diabase on the
Continent, is a holocrystalline mixture of augite, labradorite,
felspar, and titaniferous iron ore, or magnetite. Its effusive
equivalent is basalt; gabbro forms its plutonic roots. It
appears to have been a subterranean intrusion of molten
70 GEOLOGY UF TASMANIA.
material, which never succeeded in reaching the surface, or
if it did, its superficial, subaerial portion has been renioved
by denudation. The masses now visible, as at Mount Wel-lington, and crowning the Tiers, may be looked upon as hugelaccolites and sills. Up to the present, no evidences of lava
flow have been found in the structure of this rock. It is
devoid of ore-deposits.
Tertiary.
A great stratigraphic break exists between the Mesozoicand the succeeding strata. The Tertiary system cannot besubdivided as in Europe. Mr. E.. M. Johnston has pro-
posed the two divisions, palaeogene and neogene, which are
here adopted. According to this arrangement, the Ter-
tiaries will be subdivided, as follows :—
Neogene ( =: approximately to pliocene)
—
4. Glacier moraines of the Western highlands.
River terraces and estuarine deposits.
Paleogene ( = Eocene to miocene)
—
3. Basalt lavas.
2. Fluviatile and lacustrine clays and sands, tin-ore
drifts and leads.
1. Fossiliferous marine beds at Table Cape (= Eocene).
1. The researches of J. Dennant and the late Professor
Ralph Tate have shown the marine fossiliferous beds at
Table Cape to be of Eocene age. These strata are covered
with the basalt, which, in the Island, appears to separate
the lower from, the upper Tertiaries.
2. The extensive lacustrine deposits within the watershed
of the Tamar and its tributaries were described long ago byMr. Johnston, under the apt title of sediments of the Laun-ceston Tertiary basin. They cover an area of 600 square
miles, and embrace the pre-basaltic or palaeogene clays andsands, which are spread all over that part of the Island, as
well as the post-basaltic, or neogene, valley terraces. Thetliickness of these beds is from 900 to 1000 feet.
At Launceston, the ferruginous sands and clays of the
Windmill Hill are palaeogene. They contain fossil impres-
sions of the plant genera, Betula, Fagus, Quercus, Cinnamo-rnum, Banksia. At Dilston. Windermere, and MuddyCreek similar beds occur. At Carr Villa, the boring-core
showed an impression of Betida at a depth of 500 feet. Abore at Belmont went down in the palaeogene sandstones and
I
GEOLOGY OF TASMANIA. 71
shales to 894 feet, without reaching bottom. This is equi-
valent to about 200 feet below sear-level. At Beaconsfield,
palaeogene clays rest in a gutter of palaeozoic rocks, 270 feet
below sea-level, and their lowest layer is rich in fossil fruits
{Sfondylostroh'us, Platycoila, Cordia, &c.), and a leaf of
Ginnartiomu'rn has been recorded. Fossil conifers are also
found in this bottom clay. In the N.E., the highplateau of sand and gravel, containing alluvial tin-ore, nearDerby and Branxholm, which is capped with basalt, marksthe ancient course of the Ringarooma River before it waschoked with lava, and diverted to its present channel. AtBurnie, in a white pipeclay below the basalt, imprints of
leaves of European types have been found. At Waratah,leaf-imprints have been obtained from a greyish Tertiary
sandstone, beneath 45 feet of basalt, at a height, of 2000feet above sea-level. These leaves, have been determinedby Mr. R. M. Johnston, as follows :
—Eucalyptus kayseri
U.S., Laurus spre?itii ?i.s., Quircus hischoffensis n.s., UlmustasmanicuSy Cycadites microphylla n.s. Leaf-beds of similar
age, and containing impressions of Cinnamonmm, also occurin the cliff at Strahan. Tertiary leaf-beds also exist in thetin-drift in Thureau's deep lead at St. Helens. The basin
of the Derwent exhibits a series of Tertiary sands and clays,
the latter of which, at Cornelian Bay, Sandy Bay, One TreePoint, Glenora, &c., contain the usual impressions oiQuercus,
Fagus, Salix, Cinnamomum, &c. The so-called travertin,
at Geilston, contains Cypris cdhuryana (Johnston), conifer
stumps, and leaf impressions of Quercus, Fagus, Salix, Cin-
namom itm. At the head of Oyster Bay, near Swansea,there are Tertiary, probably palaeogene, clays, v/hich contain
a good deal of clay ironstone. Beds and seams of lignite
occur at Dilston, Evandale Junction, Kelly Basin, and other
places in Tertiary areas. At Kelly Basin, such beds con-
tain fossil resin, and at Evandale Junction the beds also en-
close resin globules.
3. At the close of the Palaeogene, a great outpouring of
basaltic lava took place, and this rock is very general
throughout the Island, though rarer on the West Coast.
Three types of basalt have been met with up to the pre-
basalt; 3, melilite basalt, associated with nephelinite. Thefirst type is the common variety of the Island. It has over-
spread the Campbell Town and Conara plains, and widely
conceals sediments in the Launceston Tertiary basin. Its
mineral constituents are uniformly felspar 4- augite -j-
olivine. Its texture is doleritic on the coast N. of Le-
froy, at Mount Horror, at Paddy's Peak, Hampshire. Fine
72 GEOLOGY OF TASMANIA.
columnar structure may be seen in the quarry near the
breakwater at Burnie. Dykes of this basalt traverse the
granite at Lottah, and at the summit of the Blue Tier. AtOne Tree Point, Sandy Bay, a basalt is exposed which con-
tains the red-iron olivine fayalite, visible under a hand-lens,
as dark red spots (described by O. E. White and W. A.Macleod). Basalt-glass, or tachylyte, occurs in the basalt
in several parts of the Island, e.g., Waratah, Richmond,Bothwell, &c, No craters are known.The second variety of basalt is that forming the remark-
able bluffs at Circular Head and Table Cape. The late
Professor Ulrich at one time determined it to be nepheline-
bearing, but afterwards withdrew the reference to nepheline,
believing the mineral in question to be apatite. Apatiteis abundant in the rock, but recently microscopical examina-tion has shown nepheline to be present also. The structure
is doleritic ; the mineral constituents are plagioclase -|-
augite -f- olivine + nepheline.
The third type is melilite basalt, with typical nephelinite,
or nephelinite-dolerite, at the Shannon Tier, near Bothwell.
The geological horizon has not yet been determined, but the
age is believed to be Tertiary.
4. Neogene.—The post-basaltic valley terraces can only
be separated from the earlier Tertiaries by position andlithological characters. Some of the gravel drifts of theDerwent, of the Longford plain, and in the neighbourhoodof Launceston, belong to this division. The lignite bedsof the Henty liiver contain leaves of Fagiis jonesii, (John-ston) and Acacia rneiringii (Johnston), both closely re-
sembling existing species.
The close of the Tertiary, or the beginning of the Quar-temary, witnessed a glacier epoch in the western part of
the Island. The highlands round Barn Bluff, Mounts Tyn-dal, Sedgwick, Jukes, Darwin, &c., and the western edge of
the great central plateau, abound with tarns, ice-scratched
stones, and moraines. Signs of ice-action have been tracedto sea-level on the West Coast, but the most abundant evi-
dence is to be found above the 2000-feet level. No proofof glacier conditions m this period in the Eastern part of
the Island has been adduced yet.
Tin-ore and gold-ore are the most important of themineral resources of the Tertiary system. These occur in
the alluvial gravels and leads of the period. The sands in
the Savage River, and other tributaries of the Pieman, havebeen worked for osmiridium, and, at Mount Stronach, for
monazite. The zircon sand, near Table Cape, was also ex-
ploited a few years ago. Tertiary clays are used largely
GEOLOGY OF TASMANIA. 73
for brick-making and pottery; the gravels for road-making.Though there has beem great volcanic activity, there areno signs of Tertiary lode-deposits.
Qvarternary.Recent—
3. River alluvium and sand dunes.
2. Raised beaches and helicidcc sandstone.
Pleistocene—1. River drifts.
The later terrace drifts in the valleys of existing rivers are
referred to the Pleistocene. Sand dunes, consolidated to
shelly sandstones, occur on Cape Barren, Badger, Kangaroo,and other islands in Bass' Straits, containing shells of helix,
succinea, &c. These sandstones sometimes overlie a raised
beach. The raised beaches on the North Coast indicate
elevation within the recent period.
The foregoing sketch does not pretend to do more thanmerely outline the general geology of the Island. Muchinformation has been drawn from the labours of Mr. R. M.Johnston, here acknowledged, but many important matters
still require attention. Among these are—(1) age of the
schists of Mounts Lyell and Read; (2) age of the horn-blende schists of the Rocky River; (3) age of the quartz-
porphyry, or felsite, of Mounts Jukes and Darwin, and its
relation to the granite; (4) age and nature of the BarnBluff schists
; (5) age of the gneiss and schists of the UpperForth > (6) connection (if any) of the Lynchford augite-
syenito-porphyry with the felsites on Mount Jukes andMount Read
; (7) geological occurrence of the Mesozoic dia-
base; (8) origin of the obsidian '' buttons "
; (9) connection
of the nepheline basaltoid rocks at Circular Head and TableCape with the prevailing normal basalts; (10) age of the
elseolite syenites and phonolitic rocks at Port Cygnet andtheir boundaries; (11) origin of the lake basins in glacier
areas and on the Tiers; (12) the nepheline and melilite
rocks at Shannon Tier; (13) the occurrence of garnetiferous
chlorite schist in granite at St. Helens, and numerous other
questions fraught with interest to the geologist.
These remarks may be closed by mention of the names of
resident geologists, to whom inquirers may address theim-
selves for information :—In the South : Mr. R. M. John-ston, stratigraphy and palaeontology; Mr. Thos. Stephens,
M.A., Hobart, stratigraphy; Mr. O. E. White, Hobart,
74 UEOLOGY OF TASMANIA.
eruptive rocks. In the NortK.: Mr. W. F. Petterd, Laun-ceston, mineralogy and petrology of the I&land; Mr. Geo.
A. Waller, Assistant Government Geologist, Launceston,
geology and ore deposits. In the West: Mr. F. J. Ernst,
Zeelian, eruptive rocks and ore deposits; Mr. T. B. Moore,
Strahan, glacial geology. In the East : Mr. Henry Grant,
St. Helens, granites and tin-ore deposits.
Suites of Tasmanian rocks, fossils, ores, and minerals maybe seen at the Tasmanian Museum, Hobart (Mr. Alex.
Morton. F.L.S., Curator) ; the Victoria Museum, Launces-
ton (Mr. H. H. Scott, Curator) ; the Zeehan School of
Mines (Mr. Reid, Director) ; the Government Geologist's
Office, Launceston. A complete collection of Tasmanianminerals is owned by Mr. W. F. Petterd, Launceston.
